1. Field of the Invention
This invention relates to a leakage measuring apparatus which determines whether there is a leak in a vessel or parts which are required to be air-tight.
2. Description of Related Art
An example of conventional art of this type of leakage measuring apparatus is disclosed in JP-A-55-63732.
The apparatus described in the above-mentioned Japanese Patent Application comprises means for applying the same fluid pressure to both an object to be measured and a reference tank, a differential pressure sensor for detecting a differential pressure between the object to be measured and the reference tank, a capacity variable portion communicating with the object, and control means for controlling the capacity variable portion so as to reduce the differential pressure to zero in response to output of the differential pressure sensor, wherein the amount of leakage of the object under leak test is measured regardless of the inner volume of the object, a difference in the internal pressures of the object and the reference tank is measured, and then the control means adjusts the capacity of the capacity variable portion to reduce the differential pressure to zero. The amount of leakage is generally very little and the capacity of the capacity varying portion may be small, so that the whole apparatus can be reduced in size. Other features of this apparatus are that the apparatus can be formed in a robust manner, and that the operation is made stable by use of feedback control by detecting the differential pressure and adjusting the capacity of the capacity variable portion to reduce the differential pressure to zero.
However, the principle of the conventional leakage measuring apparatus is applying the same fluid pressure to both the object under leak test and the reference tank, detecting the differential pressure between the object and the reference tank with an electric signal through a differential pressure detector, and determining the displacement of the piston (capacity variable portion) on the test fluidic circuit side so as to estimate the leakage. In an actual measurement of leakage, the leakage from the object under test is usually small and the volumetric change may be extremely small, so that errors in measurement of the piston displacement must be minimized for accurate measurement of the leakage. Particularly, when leakage of the object is very small, a long time is required for measurement of leakage in the object, that is, the inspection efficiency is low. In order to increase the displacement of the piston relative to the amount of leakage, it is necessary to reduce the diameter and increase the length of the piston, but manufacturing such a piston is difficult.
The above conventional apparatus has an undesirable time lag although a differential pressure sensor controls the piston communicating with the object under test, and a low signal-to-noise ratio owing to the effects of the distortion in the object to be measured and the reference tank derived from the applied pressure. Thus with the conventional apparatus, the accuracy of measurement is not sufficient.